Bone is a dynamic tissue, and homeostasis requires a balance between the formation of new bone and the resorption of previously formed bone. Calcitonin, a peptide hormone secreted by the thyroid and thymus, plays an important role in maintaining bone homeostasis. Calcitonin binds to osteoclasts, cells in the bone tissue which mediate bone resorption. Calcitonin immobilizes osteoclasts, thus inhibiting bone resorption with a resultant decrease in the amount of calcium released by bone into the serum. The inhibition of bone resorption has been exploited by using calcitonin as a treatment for osteoporosis.
The calcitonin receptor is believed to be a member of the G-protein coupled receptor family. It has been demonstrated that activation of the receptor results in stimulation of two independent intracellular pathways, the cyclic AMP and inositol triphosphate pathways (Chabre et al., Molec. Endocrin. 6(4):551-556, 1992). The cloning of the parathyroid hormone, porcine calcitonin, secretin and glucagon receptors has established the possibility that a new family of G protein-coupled receptors exists. These receptors show little homology to the previously known G protein coupled receptors that included the beta adrenergic receptor, the serotonin receptor and the glutamate receptors.
At the present time, salmon calcitonin is preferred over human calcitonin for treatment of osteoporosis. The worldwide market for salmon calcitonin exceeds $500 million annually. Salmon calcitonin has been shown to be considerably more effective in arresting bone resorption than human forms of calcitonin. There are several hypotheses for why salmon calcitonin is more potent than human calcitonin in treatment for osteoporosis. These hypotheses include: 1) salmon calcitonin is more resistant to degradation, 2) salmon calcitonin has a lower metabolic clearance rate (MCR) and 3) salmon calcitonin may have a slightly different conformation, resulting in a higher affinity for bone receptor sites.
Despite the advantages associated with the use of salmon calcitonin for treatment of osteoporosis in humans, there are also disadvantages. The average cost can exceed $200 per month, and treatment involves prophylactic administration for 5 or more years. Another problem is that, in the United States, calcitonin must be administered by injection. In addition, some patients develop antibodies to non-human calcitonin. Therefore, new analogs of salmon or human calcitonin that are potent inhibitors of bone resorption, less expensive, more convenient to administer and non-immunogenic are needed.
The discovery and testing of possible compounds for use as calcitonin analogs require high through-put screening systems. Such a system would preferably use a cellular target, such as a cultured cell line containing high levels of the appropriate calcitonin receptor, to identify and measure responses to putative analogs. Quite surprisingly, the present invention provides human calcitonin receptors for use in screening systems for identifying calcitonin analogs, and fulfills other related needs.